The present invention generally relates to a gas detecting device, and particularly to a gas detecting device which has four terminals and is suitable for a gas leak alarm designed to detect gas such as LP gas and commercialized gas. Further, the present invention relates to a gas detecting system which employs the 4-terminal gas detecting device.
Referring to FIG.1, there is illustrated a gas detecting system including a conventional gas detecting device 100. A pair of heater coils 112 is covered by a sintered ceramics member 115 of a size of 2 mm=2 mm to 3 mm=3 mm, which is formed of a metal oxide semiconductor. The sintered ceramics member or gas sensitive member 115 is sensitive to gas. The heater coils 112 are connected to four terminals 113, which penetrate a base 111. The four terminals 113 are connected to a variable power source 116, a constant power source 117 and a resistor R1. During a gas detecting operation, the gas sensitive member 115 is heated. When the gas sensitive member 115 reacts to gas, the resistance of the gas sensitive member 115 decreases due to gas absorption occurring on an exposed surface of the gas sensitive member 115. A variation in resistance of the gas sensitive member 115 is extracted as a variation in voltage across the register R1.
However, the contact resistance between the gas sensitive member 115 and the pair of the heater coils 112 depends on a condition of contact formed therebetween. The occurrence of variation in contact resistance prevents accurate gas detection.
An improved gas detecting device has been proposed in Japanese Laid-Open Patent Application No. 61-191953. The proposed gas detecting device includes a substrate, an insulator, a pair of detection leads, a passivation layer, and a gas sensitive layer. The pair of detection leads is arranged on the insulator so that an end of each of the detection leads is opposite to each other. The detection leads are partially covered by the gas sensitive layer at the opposed ends thereof. The rest of each of the detection leads is covered with the passivation layer.
However, even by the proposed structure for the gas detecting device, the gas detection signal varies depending on the contact resistance formed between the detection leads and the gas sensitive layer.
The inventors have proposed a further improved gas detecting device in U.S. patent application Ser. No. 07/288,279 filed on Dec. 22, 1988, the disclosure of which is hereby incorporated by reference. However, even by the proposed structure, it is difficult to completely remove the influence of variation in contact resistance.